The present invention relates to material handling apparatus and more particularly, to a lightweight portable conveyor having parallel endless belt segments with upstanding cleats for lifting loads of shingles, tiles tar paper and other building materials to rooftops.
Residential construction frequently requires the lifting of substantial amounts of building materials to rooftops under construction or repair. It is inefficient and dangerous to accomplish such lifting with manual labor. The high cost of operating forklifts, cranes and other heavy lifting equipment is prohibitive is such applications. Skilled labor is required to operate such machinery and the hourly rental cost of the heavy lifting equipment can be very substantial.
Conveyors have long been available for lifting materials from the ground to higher elevations. If one single wide belt is utilized, the conveyor must have a very substantial supporting framework due to the substantial weight of the belt. Such single wide belt conveyors are thus quite heavy and not adapted for portability between job sites. In order to lessen the weight of lift conveyors various models have been developed that employ a long extensible boom with a pair of pulleys mounted at the rearward or lower end of the boom and another pair of pulleys mounted at the forward or upper end of the boom. A pair of endless belts, each having upstanding longitudinally spaced load supporting metal cleats, are then entrained around each set of forward and rearward pulleys so that they can be driven around the pulleys in parallel. An electric motor drives the rear pulleys through a gear reduction mechanism. Each belt is made of a light weight but very strong woven fabric material. Individual loads of building materials can be stacked on top of the upper forward moving segments of the belts, one load in advance of each set of transversely aligned cleats. Due to tolerance variations, the pulleys do not have identical diameters and the belts do not have identical lengths. Therefore, even though the cleats are initially aligned in the transverse direction, over time the cleats on one belt gradually lag further and further behind the corresponding cleats on the opposing belt. When the misalignment of corresponding opposite cleats becomes too great loads cannot be properly retained by the cleats and they fall off of the conveyor.
It is not practical to use chains instead of belts to ensure that the opposing pairs of cleats stay aligned. This is because such conveyors typically extend over thirty feet in length and the weight of such long endless chains would be prohibitive, not to mention the problems with rust and breakage of the chains that would be expected in harsh construction site environments. One solution to the aforementioned problem is disclosed in U.S. Pat. No. 4,366,900 granted in 1983 to Johansson. It discloses a lightweight twin belt lift conveyor which includes a belt synchronizing mechanism to maintain the load supporting cleats in opposing transverse alignment. Magnetic position indicators on the belts are sensed by a control circuit which selectively actuates clutch mechanisms to briefly stop either rearward drive pulley so that the other belt can catch up. However, the clutch mechanisms are complex, expensive, and subject to failure from excessive loading and wear. Also, the momentary stoppage of the drive pulleys can be abrupt and dislodge the loads.
U.S. Pat. No. 4,582,192 granted in 1986 to Rojlar, assigned to the inventor of the subject application, discloses another lightweight lift conveyor with a less complex belt synchronizing mechanism. Reed switches sense magnetic position indicators on the belts A control circuit connected to the reed switches momentarily moves spacer arms into and out of engagement with the drive pulleys to vary their circumference and thereby momentarily vary the speed of advancement of the corresponding belt. Again this system, while serviceable, was too expensive, too complex and subject to failure in the harsh construction site environment. A variation on this conveyor was successfully commercialized for many years by the assignee of the present application in which the operator manually moved a spacer into engagement with one of the drive pulleys as needed. However the operators tended to forget to engage or disengage the spacer. In addition, some operators actually used the spacers as brake mechanisms, causing damage and/or interfering with the proper operation of the lift conveyor.
U.S. Pat. No. 4,854,447 granted in 1989 to Johansson discloses a light-weight lift conveyor system having a single endless belt that is entrained about a pair of rearward pulleys and a pair of forward pulleys to define a pair of parallel upper forward run segments and a pair of lower rearward running return segments that criss-cross. With this arrangement, the load-supporting cleats remain transversely synchronized, i.e., side-by-side. This eliminates the need to use side-by-side, separate parallel belts and complex speed controls, clutching arrangements and/or pulley enlargers in order to transversely synchronize the cleats. In the design of the aforementioned U.S. Pat. No. 4,854,447 of Johansson, the lower return run segments of the belt criss-cross underneath the longitudinal frame spar or boom. This results in several problems. First of all, the cleats can tangle with each other as they cross each other. This can bend or even tear off the cleats, sever the belt, or damage the belt drive mechanism. Secondly, the underside of the conveyor cannot be leaned against, and supported by, a structure such as a roof or a gutter. Some sort of support stand must be attached to the remote outer end of the boom that will extend below the boom and the return segments of the belt. This type of stand adds weight, and also results in the load carried by the forward upper segments of the belt being delivered to a greater height than is necessary or convenient. Thirdly, it has proven to be extremely difficult for a construction worker to properly mount the endless belt at the job site.